2001, Robert Stockwell, Donka Minkova, English Words: History and Structure, page 104,

One of these processes, the process of T-Lenition, is extremely common, even though it takes place only when the input consonant is adjacent to a small number of affixes. In this change, a stopped consonant, [p t k b d g], becomes a fricative, [s, z, š, ž]. This process is called lenition, or weakening.

Environments are an essential part of any discussion of lenition. Textbooks often describe lenition as occurring in the weak intervocalic or word-final environments. The canonical examples of lenition given earlier in (1) through (3) all occur either between vowels or between sonorants.